I'm abstaining from this poll. Frankly, I can't stand any of the Mario Kart games anymore. Despite fond memories of Battle Mode in the SNES and N64 Karts, I've tried to go back and replay them fairly recently and they just bore me. The post-N64 Mario Karts are won largely by luck (I'll give Activision credit for one thing in that silly Blur commercial: Mario Kart isn't about racing anymore, but about "making friends."), so I don't care for those either.
I will say, though, that Mario Kart DS probably has my favorite tracks of any Mario Kart game, especially Waluigi Pinball and whatever level you flew through Bowser's Airship Fleet.